Looking at stuff on the moon: an analogy - The online computery journal thingy of a turtle

Jan. 1st, 2009

02:04 pm - Looking at stuff on the moon: an analogy

Some people have wondered why we can't simply prove to the moon hoax conspiracy crowd that we've been to the moon, by pointing a telescope at the moon and photographing the artifacts left there by the Apollo missions: the LEM landing gear, the flag, the moon rover, and so on.

The simple answer is that we don't have a telescope powerful enough. This strikes some people as counterintuitive; we can see stars and nebulas millions of light-years away, can't we? But it's not how far away the thing is, it's how much of the visual field it covers. And the Apollo detritus is just way too small.

I'm trying to think of a good analogy to explain this. Here's what I've got so far:

Imagine you're standing on an observation point at the top of a hill at night. You can see the lights of a distant city. Maybe you can even identify some; for instance, that blinking red blob is the casino on Main Street, or that purple dot is the movie theater.

Now look down at your thumb. There are bacteria crawling around on your skin. Can you see them?

The city lights are the stars, your thumb is the moon, and the bacteria are the astronaut artifacts.

Comments:

That's about right. The problem isn't faintness of light (in fact there are neutral density lunar filters so that looking at the moon with a telescope isn't so blinding), but a matter of resolving such a tiny image.

Or consider the difference between seeing the far off lights and being able to read the too fine print on a poorly designed bumper sticker.

This is also why the old conspiracy theory about being able to read license plates with spy satellites is fiction. (Well, and also because license plates don't face upwards.)

People overestimate the resolution of spy satellites, partly because of what they see on Google Earth. What they don't realize is that the really high-res images on Google Earth are taken from airplanes, not satellites.

Since the biggest thing left behind is the lunar lander itself, and using the landing gear span (the biggest distance) that's just over 9 meters. And the moon is on average 384,403 km away from the earth, a bit of trig and fiddling about gives 0.0005 (rounding up) arc seconds. So to detect that (not image it, merely detect as a fuzzy blob) would require a telescope with a primary of 2400 cm, or about 945 inches.

And that ignores atmospheric effects, that the earth-moon distance is rarely the average, and that the result is only enough to say "Hey, we found a blob where we think a blob ought to be." and not a nice pretty overhead view of an Apollo Lunar Module.

Even if we could get images as clear as a typical National Geographic nature photo, there'd be the difficulty of positioning the telescope exactly right. How long would we have to sweep it around the surface of the moon before we found the landing site again? You'd have to move the scope very slowly, too.

There was a Mythbusters where they went to an observatory.Long story short we put a reflector on the moon, about human-sized-tall. They point a laser at the moon.They fire it. It shoots the light right back into a receptor.